The basic architectural unit of the liver is the liver lobule. Each lobule consists of plates of hepatocytes lined by sinusoidal capillaries that radiate toward a central efferent vein. Liver lobules are roughly hexagonal with each of six corners demarcated by the presence of a portal triad (portal vein, bile duct, and hepatic artery). Although hepatocytes are the major parenchymal cell type of the liver they function in concert with cholangiocytes (biliary epithelial cells), endothelial cells, sinusoidal endothelial cells, Kupffer cells, natural killer cells and hepatic stellate cells. This complex architecture is crucial for hepatic function.
The existence of liver stem cells remains controversial. On one hand, tissue maintenance in the liver and liver regeneration upon certain types of injury, are not driven by stem cells but rather by division of the mature cells (hepatocytes or cholangyocytes). However, liver injury models in which hepatocyte proliferation has been inhibited also demonstrated the ability of the organ to regenerate in response to damage. This suggests that the liver can be considered as an organ with facultative stem cells.
So far, liver cultures derived from hepatocytes, or by differentiation of Embryonic Stem cells (ES) or induced Pluripotent Stem Cells, do not expand and self renew for longer periods.
Recently, in the small intestine, the gene Lgr5 was identified which is specifically expressed in cycling Crypt Base Columnar (CBC) cells, which are small cells that are interspersed between the Paneth cells (Barker et al., 2007. Nature 449: 1003-1007). Using a mouse in which a GFP/tamoxifen-inducible Cre recombinase cassette was integrated into the Lgr5 locus, it was shown by lineage tracing that the Lgr5+ CBC cells constitute multipotent stem cells which generate all cell types of the epithelium even when assessed 14 months after Cre induction.
It was recently discovered that also Lgr6, besides Lgr5, but not Lgr4, is a unique marker for adult stem cells. While Lgr5 is expressed in stem cells of brain, kidney, liver, retina, stomach, intestine, pancreas, breast, hair follicle, ovary, adrenal medulla, and skin, Lgr6 is expressed in stem cells of brain, lung, breast, hair follicle, and skin.
Here we have developed a method to culture adult liver progenitors and to obtain a liver organoid that shows longer-lived maintenance, are able to differentiate to both, hepatocyte and cholangiocyte lineages and preserve the basic physiology of isolated liver fragments.